A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction), while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In a lithographic apparatus, the size of features that can be imaged onto the substrate is limited by the wavelength of the projection radiation. To produce integrated circuits with a higher density of devices, and hence higher operating speeds, it is desirable to image smaller features. While most current lithographic projection apparatus employ ultraviolet light generated by mercury lamps or excimer lasers, it has been proposed to use shorter wavelength radiation, in the range of 5 to 20 nm, in particular around 13 nm.
Such radiation is termed extreme ultra violet (EUV) or soft X-ray and possible sources include, for example, laser produced plasma sources, discharge plasma sources, or synchrotron radiation from electron storage rings. These types of radiation require that the beam path in the apparatus be evacuated to avoid beam scatter and absorption. Because there is no known material suitable for making a refractive optical element for EUV radiation, EUV lithographic apparatus must use mirrors in the radiation (illumination) and projection systems. Even multilayer mirrors for EUV radiation have relatively low reflectivities and are highly susceptible to contamination, further reducing there reflectivities and hence throughput of the apparatus. This may impose further specifications on the vacuum level to be maintained and may necessitate especially that hydrocarbon partial pressures be kept very low.
In a typical discharge plasma source, plasma is formed by an electrical discharge. The plasma may then be caused to compress so that it becomes highly ionized and reaches a very high temperature, thereby causing the emission of EUV radiation. The material used to produce the EUV radiation is typically xenon or lithium vapor, although other gases, such as krypton or tin or water, may also be used. However, these gases may have a relatively high absorption of radiation within the EUV range and/or be damaging to optics further downstream of the projection beam and their presence should therefore be minimized in the remainder of the lithographic apparatus. A discharge plasma source is disclosed, for example, in U.S. Pat. No. 5,023,897 and U.S. Pat. No. 5,504,795, both of which are incorporated herein by reference.
In a laser produced plasma source, a jet of, for example, (clustered) xenon may be ejected from a nozzle, for example, produced from an ink-jet like nozzle as droplets or thin wire. At some distance from the nozzle, the jet is irradiated with a laser pulse of a suitable wavelength for creating a plasma that subsequently will radiate EUV radiation. Other materials, such as water droplets, ice particles, lithium or tin, etc. may also be ejected from a nozzle and be used for EUV generation. In an alternative laser-produced plasma source, an extended solid (or liquid) material is irradiated to create a plasma for EUV radiation. Laser produced plasma sources are, for example, disclosed in U.S. Pat. No. 5,459,771, U.S. Pat. No. 4,872,189, and U.S. Pat. No. 5,577,092, all of which are incorporated herein by reference.
During generation of EUV radiation, particles are released. These particles, hereinafter referred to as debris particles, include ions, atoms, molecules, and small droplets. These particles should be filtered out of the EUV radiation, as these particles may be detrimental to the performance and/or the lifetime of the lithographic apparatus, in particular the illumination and projection system thereof.
International Patent Application Publication No. WO 99/42904, incorporated herein by reference, discloses a filter that is, in use, situated in a path along which the radiation propagates away from the source. The filter may thus be placed between the radiation source and, for example, the illumination system. The filter includes a plurality of foils or plates that, in use, trap debris particles, such as atoms and microparticles. Also, clusters of such microparticles may be trapped by these foils or plates. These foils or plates are orientated such that the radiation can still propagate through the filter. The plates may be flat or conical and may be arranged radially around the radiation source. The source, the filter and the projection system may be arranged in a buffer gas, for example, krypton, whose pressure is about 0.5 torr. Contaminant particles then take on the temperature of the buffer gas, for example, room temperature, thereby sufficiently reducing the particles velocity before the end of the filter. This enhances the likelihood that the particles are trapped by the foils. The pressure in this known contaminant trap is about equal to that of its environment, when such a buffer gas is applied.
International Patent Application Publication No. WO 03/034153, incorporated herein by reference, discloses a contaminant trap that includes a first set of foils and a second set of foils, such that radiation leaving the source first passes the first set of foils and then the second set of foils. The plates, or foils, of the first and second set define a first set of channels and a second set of channels, respectively. The two sets of channels are spaced apart, leaving between them a space into which flushing gas is supplied by a gas supply. An exhaust system may be provided to remove gas from the contaminant trap. The pressure of the gas and the space between the two sets of channels may be relatively high so that debris particles are efficiently slowed down, further enhancing the likelihood that debris particles are trapped by the second set of foils. The first and second set of channels provide a resistance to the gas when the gas moves from the space between the two sets of channels in the channels of either the first or the second set. Hence, the presence of the gas is more or less confined to the space between the two sets of channels.
Even though the platelets or foils are positioned such that radiation diverging from the radiation source can easily pass through the contaminant trap, the foils or platelets do absorb some EUV radiation and, therefore, some heat. Moreover, these foils are heated by colliding debris particles. This may result in a significant heating of the foils and heating of a supporting structure that supports the foils. This may lead to thermal expansion of the foils and of the supporting structure. As optical transmission of the contaminant trap is very important in a lithographic apparatus, the deformation of a foil due to thermal expansion of the foil should be minimized.
European Patent Application Publication No. EP 1 434 098 addresses this problem by providing a contamination barrier, i.e. a foil trap or contaminant trap, that includes an inner ring and an outer ring in which each of the foils or plates is slidably positioned at least one of its outer ends in grooves of at least one of the inner ring and outer ring. By slidably positioning one of the outer ends of the foils or plates, the foils or plates can expand in a radial direction without the appearance of mechanical tension, and thus without thermally induced deformation of the plate or foil. The contamination trap may include cooling means arranged to cool one of the rings to which the plate or foils are thermally connected.